1. Field of the Invention
The present invention relates to an aromatic imide polymer film for a magnetic recording material and a process for producing the same.
More particularly, the present invention relates to an aromatic imide polymer film having a front face-side surface having a relatively high surface smoothness, which is suitable for forming a magnetic recording layer thereon, and a back face-side surface having a high sliding property, which is useful as a heat resistant base film for a magnetic recording material, and a process for a producing the same with a high productivity.
2. Description of the Related Art
It is known that aromatic imide polymer films have a high mechanical strength and an excellent heat resistance and thus are useful as a base film on which a magnetic layer consisting of, for example, a Co-Cr alloy is formed by a vacuum vapor-depositing method or a sputtering method.
The conventional aromatic imide polymer film however, does not have satisfactory sliding properties, and an attempt was made to provide a sliding film containing an inorganic lubricant or filler therein and having a number of fine convexities formed on the surfaces thereof. In this type of sliding film, however, the two surfaces thereof were roughened by a number of fine convexities formed thereon due to the inorganic filler particles, and thus when a magnetic layer is formed on one surface of the film, the resultant magnetic recording sheet cannot stably exhibit a satisfactory output for a magnetic recording.
In another attempt at solving this problem, Japanese Examined Patent Publication (KokOku No. 55-2193 discloses a method of producing an aromatic polyimide film having a coating layer consisting of another polymer formed on one surface of the film. In this method, a surface of a polyamic acid film is coated with a solution of another polymer to form a two layer composite polyimide film.
In still another attempt, Japanese Unexamined Patent Publication (Kokai) No. 63-29700 discloses a method in which a surface of a polyimide film is coated with a solution of a polymer containing fine particles of a lubricant, and the resultant solution layer is dried and heat treated to provide a coating layer having an enhanced sliding property.
Nevertheless, this type of method of producing a high sliding coating layer on a surface of a polyimide film is disadvantageous in that undesirable impurities contaminate the coating layer; the resultant coating layer is very thin and thus cannot make smooth the surface of the polyimide film having a number of connexities formed thereon; the resultant coated film is frequently curled upon bending; and the resultant coated film exhibits an unsatisfactory smoothness on the opposite surface thereof.
Under the above-mentioned circumstances, there is a need for an aromatic imide polymer having a high sliding surface and a smooth surface thereof, which is useful as a base sheet for a magnetic recording material, and a process for producing the same with a high reproductivity.